[CT-3480] Remove the "test" Directory

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

It is confusing that we have both "test" and "tests" directories. We should remove the older "test" directory, retiring or moving the few things left there to more logical locations.

### Acceptance criteria

There is no more test directory.

### Impact to Other Teams

None.

### Will backports be required?

No.

### Context

_No response_